A conventional gas turbine engine fan includes a rotor disk having a plurality of circumferentially spaced apart rotor fan blades fixedly joined thereto. The rotor disk in one embodiment includes a plurality of circumferentially spaced apart, axially extending dovetail slots in which are respectively slidably inserted a complementary axial entry dovetail of the fan blade for securing the fan blade to the rotor disk. The axial dovetail slots allow for easy assembly of the individual fan blades axially into a respective dovetail slot. Conventional blade retainers are disposed at both forward and aft ends of the dovetails for axially retaining the dovetails in the dovetail slots during operation.
A gas turbine engine fan is typically designed to accommodate foreign object ingestion such as bird strikes against the fan blades thereof without excessive damage which might completely sever a fan blade during operation or cause excessive unbalance of the fan which would require shutdown of the engine during operation. In a gas turbine engine powering an aircraft in flight, bird strikes typically occur during takeoff or descent of the aircraft over an airport runway. Furthermore, in a blade release or blade-out occurrence, the released blade may impact an adjacent blade and impose forces like those in a bird strike. In the bird strike situation, it is desirable that the engine remain operational for providing power as required even in the event of bird ingestion thereto. And, in the blade release situation, preventing damage to the blade retention system of the adjacent blade is desirable.
Since a fan blade typically has a large amount of twist from its root to its tip, a bird strike typically impacts the aft facing or pressure side of the fan blade at high rotational speed thereof during takeoff, for example, which produces impact loads in the fan blade. At relatively low speeds of the fan blade which may occur during descent, for example, a bird might alternatively impact the forward facing or suction side of the fan blade.
In both situations, the bird strikes include an axial component of load at the bird strike location which may be near the fan blade tip, which axial impact load results in a corresponding axial reaction load in the blade retainers, as well as bending moments about the dovetail and root of the blades which create bending stresses therein. The forward bird impact load on the pressure side of the fan blade typically has the largest magnitude which correspondingly creates the largest magnitude axial impact reaction forces in the forward blade retainer and the maximum impact bending stresses at the blade root and dovetail. To accommodate these bird strike axial impact loads, the blade retainers must be suitably sized which increases their weight and requires a correspondingly heavier rotor disk for reacting the impact loads channeled therethrough.